@john32123666 : Thanks for the response. I gather the graphics you posted are from your own client. The only difference I see is that you have two boxes enabling DHT ticked, whereas I don't. I thought I saw some posts on here that DHT was a "bad thing," but I can easily change this.
@MrMazda : I see that several of my older "nobody's leeching" torrents have been freeleeched (I wonder who did that? ) and my client & modem are now maxed out seeding them. (Several are now fully seeded with a dozen or more other seeders, so I've paused those in order to put maximum bandwidth on a few of the very large torrents.)
So it seems that my settings and such are OK, and it was simply a matter of having no leechers on many of my torrents. There's some older stuff that I've kept live because I think it's of value and I wanted it to be available to people. I'm not concerned about ratio (I'm doing OK with that); I just wanted to be a good sharer.